Beamforming is a signal processing technique for directional signal transmission or reception. This is achieved by combining antenna elements in a phased array in such a way that signals at particular angles or directions experience constructive interference while others experience destructive interference. Beamforming can be used at both the transmitting and receiving ends in order to achieve spatial selectivity. Beamforming is for instance intended to be used in the 5th generation (5G) wireless communications system. The beamforming can be performed at different parts along the signal path, such as in radio frequency (RF), in intermediate frequency (IF), in baseband (BB) frequency or in digital domain.
Digital beamforming (DBF) has many advantages, including the ability to apply multiple sets of weight vectors simultaneously to receive multiple beams. For example, in phased array receivers, multiple beams may be created from digitalized signals after analog to digital converters (ADCs) in each channel of the receiver. However, there are some drawbacks to DBF, e.g. the need to have separate receive chains from each antenna element through the ADCs. Therefore, to create multiple beams, high speed ADCs are needed which are very power hungry. Another issue is that the data rate created from each channel is very high, and data links have to be connected from chip-to-chip or die-to-die. This may require high speed field-programmable gate arrays (FPGAs) with many input/output (I/O) pins to receive the signals from the ADCs, which leads to huge data rate and complicated data interfaces. Thus, this type of configuration requires significant power, which may lead to heat dissipation issues. Further, some DBF algorithms require so much processing that large array with many elements become impractical.
It is difficult to perform multiple beam forming in RF and IF paths. There are some multiple beamforming methods performed in baseband. For example, an antenna array with analog beamforming (ABF) capability may be utilized for receiving RF signals. The ABF array manipulates the phase and/or gain of signals arriving at each element, then sums the signals, and continues processing on the signals e.g., utilizing ADCs.
The U.S. Pat. No. 8,743,914 discloses an analog beamforming receiver for forming multiple simultaneous independent beams. The analog beamforming receiver includes a first receive element for receiving a signal, where the first receive element is coupled with circuitry for sequentially adjusting at least one of a phase or a gain of the signal received by the first receive element. The analog beamforming receiver may also include a second receive element for receiving the signal, where the second receive element is coupled with circuitry for sequentially adjusting at least one of a phase or a gain of the signal received by the second receive element. The analog beamforming receiver further includes a combiner coupled with the first receive element and the second receive element for combining the phase or gain adjusted signals from the first receive element and the second receive element to form a combined analog signal. The analog beamforming receiver also includes an analog to digital converter coupled with the combiner for converting the combined analog signal from the combiner into a digital signal. The analog beamforming receiver further includes a demultiplexer coupled with the analog to digital converter for demultiplexing the digital signal from the analog to digital converter into a plurality of demultiplexed signals forming a plurality of multiple simultaneous independent beams. In this analog beamforming receiver, a number of simultaneous beams are determined by the maximum sample rate of the ADC, the sample rate per beam and the ABF switching time.
However, analog baseband beamforming has limited accuracy in the circuitry for adjusting the phase and gain of the signals, e.g. variable gain amplifiers and/or phase rotators, especially at higher frequency due to the fact that the amplifiers are built with feedback mechanism. Usually the amplifiers consume more power as the bandwidth increases. Further, there are different DC offsets at output of the amplifier/phase rotation circuitry when the phase rotation angle changes which are problems need to be solved.